Home > Knowledge > Content
Braking resistor
Sep 13, 2017


Braking resistor, which is a kind of ripple resistance, is mainly used in the mechanical system of the inverter to control the motor quick stop, and helps the motor to convert the regenerative energy generated by the fast stop into heat energy.

Chinese name brake resistor foreign language name match resistor deny high power resistance, brake resistance

table of Contents

a ceramic tube

Is the alloy resistance wire skeleton, also has the effect of radiator;

b alloy resistance

Flat belt wave shape, wrapped around the surface of the ceramic tube, responsible for the motor's regenerative energy into heat;

c coating

Coated on the surface of the alloy resistance wire, with high temperature characteristics, the function is flame retardant;

Principle editor

First, the brake unit principle: brake unit by the high-power transistor GTR and its drive circuit. Its function is for the discharge current link capacitor in the specified voltage range can not be stored or the internal braking resistor is too low and consumption of the DC part of the "overpressure", the need for external braking components to speed up the consumption of renewable energy The

Second, the brake resistance principle: the motor in the process of falling frequency, will be in the regenerative braking state, drag the system's kinetic energy to the DC circuit, the DC voltage UD rising, and may even reach the point of danger. Therefore, the energy regenerated to the DC circuit must be consumed so that the UD is kept within the allowable range. Braking resistor is used to consume this part of the energy.

Third, the brake unit + resistance: brake unit by the high-power transistor GTR and its drive circuit. Its function is to provide a path for the discharge current IB flowing through the braking resistor.

The following is the action of the brake unit:

a, when the motor in the role of external force deceleration, the motor to run the state of power generation, resulting in renewable energy. The three-phase AC electromotive force generated by the inverter inverter part of the six freewheeling diode composed of three-phase full-bridge rectifier, so that the inverter DC bus voltage continues to rise.

b, when the DC voltage reaches a certain voltage (brake unit of the open voltage), the brake unit power switch is turned on, the current flows through the braking resistor.

c, the braking resistor releases heat, absorbs regenerative energy, the motor speed decreases, the inverter DC bus voltage decreases.

d, when the DC bus voltage drops to a certain voltage (brake unit stop voltage), the brake unit power tube off. At this point there is no braking current flowing through the resistance, the braking resistor in the natural heat, reduce their own temperature.

e, when the DC bus voltage to re-increase the brake unit action, the brake unit will repeat the above process, balance the bus voltage, the system normal operation.

As the brake unit working conditions are short-term work, that is, each time the power is very short, in the power time, the temperature rise is far from stable temperature rise; and each power after the intermittent time is longer, In the intermittent time, the temperature is enough to fall to the same as the ambient temperature, so the rated power of the braking resistor will be greatly reduced, the price also decreased; the other because there is only one IGBT, brake time ms level, Shut down the transient performance requirements of low requirements, or even require a short turn off time to reduce the shutdown pulse voltage, protection of power tube; control mechanism is relatively simple to achieve easier. Because of the above advantages, it is widely used in cranes equipotential energy load and need to fast braking but for short-term work of the occasion.

Function 

Protect the inverter

Motor in the fast stop process, due to inertial effect, will produce a lot of renewable energy, if not consumed in time part of the regenerative energy, it will directly act on

Frequency converter special type braking resistor

Frequency converter special type braking resistor

Frequency converter of the DC circuit part of the light, the inverter will report the failure, in severe cases, will damage the inverter; the emergence of brake resistor, a good solution to this problem, to protect the inverter from the motor regenerative energy hazards;

Guaranteed power supply network

The braking resistor directly converts the regenerative energy in the process of rapid braking of the motor into heat energy, so that the regenerative power will not be fed back to the power supply network without causing the voltage fluctuation of the grid, thus playing the role of ensuring the smooth operation of the power supply network.

Resistance selection 

The choice of braking resistor in addition to the inverter-specific energy consumption unit maximum allowable current limit, and the brake unit is also no clear correspondence, the resistance is mainly based on the required size of the choice of braking torque, The power is determined by the resistance and the usage of the resistor. The selection of the braking resistor resistance is a non-bremable principle: it is necessary to ensure that the current IC flowing through the braking resistor is less than the maximum allowable current output capability of the brake unit, ie, R> 800 / Ic where: 800 - DC side may occur the maximum DC voltage. Ic - the maximum allowable current of the braking unit. In order to make full use of the capacity of the selected inverter-specific brake unit, the selection of the braking resistor resistance is usually the most economical and close to the minimum value calculated above, and the maximum braking torque is obtained. Larger braking resistor power. In some cases, do not need a lot of braking torque, this time more economical way is to choose a larger braking resistor resistance, and therefore can reduce the braking resistor power, thereby reducing the purchase of braking The cost of the resistor is such that the capacity of the brake unit is not fully utilized.

Power calculation 

After selecting the resistance of the braking resistor, the power value of the braking resistor should be determined, and the selection of the braking resistor power is relatively cumbersome, which is related to many factors. The instantaneous power consumed by the braking resistor is calculated as follows: P = 7002 / R The braking resistance power calculated as above is the power value that the braking resistor can dissipate for a long period of time. However, the braking resistor Is not an uninterrupted work, this selection there is a lot of waste, in this product, you can choose the use of braking resistor, which specifies the short-term braking resistor ratio. The actual power consumed by the braking resistor is calculated as follows: P = 7002 / R × rB% rB%: Braking resistor usage. In actual use, you can follow the above formula to select the braking resistor power, can also be selected according to the braking resistor resistance and power, in turn, can calculate the brake resistor can withstand the use rate, so the correct set to avoid the braking resistor overheating And damaged.

Size calculation 

The braking torque is first estimated

Brake torque = ((motor moment of inertia + motor load measured to the measured moment of inertia of the motor) (before braking speed - after braking)) / 375 * Deceleration time - Load torque In general, There is a certain loss inside the motor, about 18% -22% of the rated torque, so the calculated result is less than this range, then no need to brake device;

Then calculate the resistance of the braking resistor

(0.1047 * (brake torque -20% of motor rated torque) Motor speed before braking) During the operation of the brake unit, the voltage of the DC link Of the lift depends on the constant RC, R is the resistance of the braking resistor, C for the inverter internal electrolytic capacitor capacity. Here the brake unit operating voltage value is generally 710V.

And then the selection of the braking unit

In the selection of the brake unit, the maximum working current of the braking unit is the only basis for selection. The calculation formula is as follows: Braking current instantaneous value = Braking unit DC bus voltage value / Braking resistor value The final calculation of the braking resistor Of the nominal power due to the braking resistor for the short-term work system, so according to the characteristics of resistance and technical indicators, we know that the nominal power of the resistance will be less than the power consumption when the power, generally available the following formula: Power = Braking Resistance Derating Factor Average Power Consumption during Braking Brake Usage%

Brake characteristics

The advantage of energy consumption braking (resistance braking) is that the construction is simple and the disadvantage is that the operating efficiency is reduced, especially when the brake is frequently consumed, and the capacity of the braking resistor is increased. [1]

Usage 

Braking resistor usage limits the efficiency of the braking resistor to avoid overheating of the braking resistor, which can affect the braking effect of the brake unit. The lower the operating rate of the braking resistor, the smaller the degree of heat generation of the resistor, the less the energy consumed on the resistor, and the worse the braking effect. At the same time, the capacity of the brake unit has not been fully utilized. In theory, the braking resistor usage rate of 100%, the use of the maximum capacity of the brake unit, the braking effect is most obvious, but this requires a greater cost of braking resistor power, the user should be considered. In the braking resistance and power have been determined under the premise of slow down the slow inertia load, select the lower resistance will achieve better results. For the need for rapid shutdown of the load, should choose a larger braking resistor usage.

Derive selection 

The braking resistor power calculated in accordance with the above method is sufficient, and further derating may be selected depending on the nature of the load.

Non-repetitive braking

The relationship between the braking power derating power and the single braking time

The relationship between the braking power derating power and the single braking time

The so-called non-repetitive braking, refers to the drag system in a very long period of time only one deceleration braking process, so the braking resistor in the period of time only one energy consumption process, the braking resistor power and therefore can be further The magnitude of the reduction and reduction depends on the impact resistance of the braking resistor and the operating time of the single deceleration brake. In the non-repetitive braking system, the relationship between the power derating of the braking resistor and the single deceleration braking time is shown in Figure 1. The braking resistance derating power and the single brake Time relationship diagram "; see, in the braking time is less than 10S case, the braking resistor power can be selected to derating to 20% or less.

Repeated braking

Relationship between Braking Resistor Power and Braking Duty Cycle

Relationship between Braking Resistor Power and Braking Duty Cycle

Some machines are required to repeatedly brake, such as lifting machinery and gantry planer, etc., in the case of repeated braking and braking time is short, the choice of braking resistor power P with the brake duty cycle (each system The ratio tb / tc of the time tb to the time interval tc between each braking is approximately linear. The smaller the braking duty cycle, the greater the magnitude of the derating of the braking resistor power (P is less). This correspondence can be understood by "graph: relationship between braking resistor power and brake duty cycle".

(Note: When braking time is longer and repeated braking, the choice of braking resistor power should be considered to leave a margin.)